Large-scale fluctuations in the distribution of galaxies from the Two Degree Field Galaxy Redshift Survey

TL;DR

This study analyzes galaxy distribution fluctuations in the 2dF Galaxy Redshift Survey, revealing large, persistent structures that challenge standard cosmological models of galaxy formation and clustering.

Contribution

It provides the first detailed statistical analysis of large-scale galaxy fluctuations, comparing observations with predictions from cosmological simulations.

Findings

Galaxy fluctuations are large and extend over significant scales.

Observed fluctuations are inconsistent with standard cosmological model predictions.

Detected fluctuations are similar across northern and southern galactic caps.

Abstract

We study statistical properties of galaxy structures in several samples extracted from the 2dF Galaxy Redshift Survey. In particular, we measured conditional fluctuations by means of the scale-length method and determined their probability distribution. In this way we find that galaxy distribution in these samples is characterized by large amplitude fluctuations with a large spatial extension, whose size is only limited by the sample's boundaries. These fluctuations are quite typical and persistent in the sample's volumes, and they are detected in two independent regions in the northern and southern galactic caps. We discuss the relation of the scale-length method to several statistical quantities, such as counts of galaxies as a function of redshift and apparent magnitude. We confirm previous results, which have determined by magnitude and redshift counts that there are fluctuations of about 30% between the southern and the northern galactic caps and we relate explicitly these counts to structures in redshift space. We show that the estimation of fluctuation amplitude normalized to the sample density is biased by systematic effects, which we discuss in detail. We consider the type of fluctuations predicted by standard cosmological models of structure formation in the linear regime and, to study nonlinear clustering, we analyze several samples of mock-galaxy catalogs generated from the distribution of dark matter in cosmological N-body simulations. In this way we conclude that the galaxy fluctuations present in these samples are too large in amplitude and too extended in space to be compatible with the predictions of the standard models of structure formation.